1. Field of the Invention
The present invention relates to polypeptides having dipeptidyl aminopeptidase activity and isolated nucleic acid sequences encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the nucleic acid sequences as well as methods for producing the polypeptides. The present invention further relates to methods of obtaining protein hydrolysates useful as flavour improving agents.
2. Description of the Related Art
Various food products, e.g., soups, sauces and seasonings, contain flavoring agents obtained by hydrolysis of proteinaceous materials. This hydrolysis is conventionally accomplished using strong hydrochloric acid, followed by neutralization with sodium hydroxide. However, such chemical hydrolysis leads to severe degradation of the amino acids obtained during the hydrolysis, and also to hazardous byproducts formed in the course of this chemical reaction. Increasing concern over the use of flavoring agents obtained by chemical hydrolysis has led to the development of enzymatic hydrolysis processes.
Enzymatic hydrolysis processes of proteinaceous materials aim at obtaining a high degree of hydrolysis (DH), and this is usually attained using a complex of unspecific acting proteolytic enzymes (i.e., unspecific-acting endo- and exo-peptidases). For example, WO 94/25580 describes a method for hydrolyzing proteins by use of an unspecific acting enzyme preparation obtained from Aspergillus oryzae. Specific acting proteolytic enzymes have not been used for this purpose because such enzymes only lead to an inadequate degree of hydrolysis.
Polypeptides having dipeptidyl aminopeptidase activity catalyze the removal of dipeptides from the N-terminus of peptides, polypeptides, and proteins. Such polypeptides are classified under the Enzyme Classification Number E.C. 3.4.14.—of the International Union of Biochemistry and Molecular Biology.
Beauvais et al. (1997, Journal of Biological Chemistry 272: 6238–6244) disclose a dipeptidyl-peptidase from Aspergillus fumigatus which has a molecular weight of 88 kDa by SDS-PAGE and a substrate specificity limited to the hydrolysis of X-Ala, His-Ser, and Ser-Tyr dipeptides at a neutral pH optimum. Tachi et al. (1992, Phytochemistry 31: 3707–3709) disclose an X-prolyl dipeptidyl aminopeptidase from Aspergillus oryzae which has a molecular weight of 145 kDa by SDS-PAGE and a substrate specificity toward the peptide bond at the carboxyl site of a proline residue in the penultimate position of N-terminal free dipeptides and amides at a neutral pH optimum.
The production of protein hydrolysates with desirable organoleptic properties and high degrees of hydrolysis generally requires the use of a mixture of peptidase activities. It would be desirable to provide a single component peptidase enzyme which has activity useful for improving the organoleptic properties and degree of hydrolysis of protein hydrolysates used in food products either alone or in combination with other enzymes.
It is an object of the present invention to provide improved polypeptides having dipeptidyl aminopeptidase activity as well as methods for obtaining protein hydrolysates with desirable organoleptic qualities and high degrees of hydrolysis.